This invention relates to a polymer latex which provides excellent long-lasting ultraviolet absorption and colorlightfastness when applied to myriad substrates, notably plastics and fabrics. The inventive latex comprises benzotriazole or benzophenone monomers copolymerized with acrylic acid comonomers, all of which are subjected to a chain transfer agent during emulsion polymerization. The resultant latex obtains excellent long-lasting ultraviolet absorption characteristics and is easy to apply to and difficult to remove from fabrics, thus providing a cost-effective improvement over the prior art. Moreover, the invention covers fabrics treated with the inventive polymer latex, particularly those fabrics which have undergone supersonic air post-treatment. The resultant articles to which the inventive latex is applied are also contemplated within this invention.
The risks posed by ultraviolet radiation have become noteworthy in recent years as concerns over environmental issues, particularly the thinning of the earth""s protective ozone layer, increase. For instance, incidences of skin cancer due to overexposure to solar radiation and thus harmful ultraviolet rays have been on the rise. Ultraviolet (UV) radiation which has proven harmful to human skin includes the two different types known as UV-A, which falls within the range of 320-400 nm along the light spectrum, and UV-B, which is between 290-320 nm in wavelength. Any manner of reducing or preventing transmission of UV light thus must effectively block or absorb such radiation between these wavelengths (290 and 400 nm).
Ultraviolet radiation also affects the color of certain substrates by initiating degradation of dyes, colorants, pigments, and the like. Long-term exposure to direct sunlight eventually results in a loss of color or, at the least, a noticeable decrease in color strength. Such effects are particularly problematic within the automotive upholstery and drapery industries since the colored surfaces, car seats and curtains, are potentially exposed to ultra violet radiation for a great deal of time.
Sun protective compositions for direct contact with skin and colored surfaces have been developed in order to better prevent the damaging results from such overexposure. For instance, PABA, or para-aminobenzoic acid, is a popular UV blocking (or absorbing) compound which may be incorporated into a composition, such as a tanning lotion for skin, or a coating composition for car seats, and the like. This compound effectively absorbs the harmful radiation within the ultraviolet range of frequencies such that the user""s skin or subject substrate is not fully exposed to such light. Recently, a new ultraviolet protective factor (UPF) test has been developed and adopted by the American Association of Textile Colorists and Chemists (AATCC) which provides a more thorough measurement of the ultraviolet absorption capabilities of specific fabrics within both the UV-A and UV-B wavelength ranges. This method, AATCC Test Method 183-1998 determines the ultraviolet radiation blocked or transmitted by textile fabrics intended to be used for UV protection. By utilizing a spectrophotometer or spectroradiometer, the UPF is calculated as the ratio of the erythemally (with erythema being the measurement of abnormal redness of skin due to ultraviolet radiation exposure) weighted ultraviolet radiation (UV-R) irradiance at the detector with no specimen present as compared to the UV-R irradiance with a specimen present, both over a range of wavelengths measured in intervals. This AATCC Test Method also measures Solar Spectral Irradiance through the subject fabric.
In order to decrease ultraviolet transmissions through clothing, past developments have provided protective measures from such harmful radiation through the introduction of certain compounds into or onto apparel fabrics. The prior art representative of this technology includes U.S. Pat. Nos. 4,857,305 to Bernhardt et al., 5,458,924 to Kashiwai et al., and 5,637,348 to Thompson et al, as well as United Kingdom Patent 889292 to American Cyanamid. Furthermore, certain types of weaves, twists, or bends of yams or fabrics have been developed which effectively screen a wearer""s skin from ultraviolet radiation. Such technology is represented within the prior art through U.S. Pat. No. 4,861,651 to Goldenhersh. With such chemically treated or physically modified fabrics, a wearer could then cover his or her skin more effectively solely through adorning themselves with such sun protective apparel. However, the prior art modified fabrics still permit transmission of relatively high levels of UV transmission and are expensive to produce. In order to decrease the potential color degradation for substrates due to ultra violet exposure, UV-absorbing copolymer latices have been utilized, most notably as films, coatings or adhesives. Such latices have provided, for example, a barrier to potentially damaging ultraviolet rays, both to a coated article and to, if such an article comprises apparel, a wearer of such an article. Polymer UV absorbing latices for textiles thus generally provide a beneficial, cost-effective protective alternative to higher density and higher costing fabrics.
Ultraviolet absorbing polymer latices incorporating UV-absorbing monomers and vinyl-functional comonomers have been disclosed within the prior art. However, nowhere has the novel procedure of emulsion polymerization of at least two monomers, all in the presence of a chain transfer agent, most notably 1-dodecanethiol, been taught, fairly suggested, or practiced. Past polymer UV absorbing latices include those taught within U.S. Pat. No. 5,629,365, to Razavi, entirely incorporated herein by reference. Such polymer latices comprise the same comonomers as in the instant, namely benzotriazole- and/or benzophenone-containing monomers polymerized with vinyl functional monomers; however, patentee both requires that the final product be subject to cross-linking, as is unnecessary within the inventive latex, and fails to mention or fairly suggest the presence of a chain transfer agent. Also of note as prior UV-absorbing polymer latices are those products taught within U.S. Pat. No. 4,528,111, to Beard et al., also herein entirely incorporated by reference. These polymers comprise the same benzotriazole comonomers as in the present invention; however, such latices are not formed of the same vinyl-functional comonomers, or through the same emulsion polymerization in the presence of a chain transfer agent as within the inventive latex. Patentee""s latices are made through a solution polymerization process which includes the utilization of environmentally damaging solvents. Furthermore, the requisite solvents used within solution polymerization procedures are known to adversely affect subject substrates, particularly textiles, through dissolving dyes and colorants, hindering lightfastness by plasticizing finishes on textiles, and degrading polymer coatings. As a result, solution polymerization is an highly undesirable method of producing a stable polymer latex, again, particularly for textile substrates.
Other prior U.S. patents disclose similar compositions and procedures as those mentioned above; however, again, there is no prior teaching of the inventive process utilizing a chain transfer agent during an emulsion polymerization in order to form an UV-absorbing polymer latex. Such U.S. patents which teach the polymerization of an UV-absorbing monomer with a vinyl functional comonomer include U.S. Pat. Nos. 3,429,852, to Skoultchi, 3,745,010, to Janssens et al., 3,761,272, to Mannens et al., 4,443,534, to Kojima et al., 4,455,368, to Kojima et al., 4,612,358, to Besecke et al., 4,652,656, to Besecke et al., 5,099,027, to Vogl et al., and 5,459,222, to Rodgers et al., all herein entirely incorporated by reference.
Even with all the previous work performed in this crowded area, there still remains a great need to produce a long-lasting, lightfast, stable, hard to remove, easily handled, and cost-effective ultra violet-absorbing polymer latex for application to certain surfaces in order to act as a barrier against potentially damaging penetrative UV rays.
It is therefore an object of this invention to provide a UV-absorbing polymer latex which produces optimum colorlightfastness performance on different substrates and effectively prevents color degradation through UV exposure to different substrates, including fabrics and polymeric films and composites. Also, it is an object of this invention to provide optimum UV-absorption performance for the inventive polymer latex through the manipulation of the specific latex monomer/comonomer ratios. Furthermore, an object of the invention is to provide the most reliable, best performing, easily adhering, and semi-permanent UV-absorbing polymer latex through the utilization of emulsion polymerization, and more particularly semi-batch emulsion polymerization, in the presence of a chain transfer agent. Additionally, it is an object of the invention to improve the overall performance of the inventive polymer latex by adjusting the post-polymerization surface tension of the latex to an optimum level on the subject substrate. Yet another object of the invention is to provide an improved UV-absorbing coating for fabrics, with improved UV-absorption and colorlightfastness, in comparison to those taught within the prior art which comprise the same type and amount of UV-absorbing monomer and vinyl-functional comonomer. Still a further object of the invention is to provide improved colorlightfastness and UV absorption for a pile fabric substrate by coating the substrate with the inventive latex and subsequently exposing the coated substrate to a supersonic air post-treatment.
Accordingly, this invention concerns a method of making an ultra violet absorbing copolymer latex by emulsion polymerization comprising
mixing together, in the presence of at least one polymerization initiator and at least one chain transfer agent,
(a) at least one monomer having at least one ultra violet absorbing functionality selected from the group consisting essentially of benzotriazole, benzophenone, and mixtures thereof; and
(b) at least one monomer having at least one vinyl functionality.
Also, the invention concerns articles made therefrom and such articles pretreated with supersonic air applications.
Nowhere within the prior art has such a specific method of producing an UV-absorbing polymer latex utilizing both emulsion polymerization and a chain transfer agent been disclosed, practiced, or fairly suggested. Such a method provides a significant advantage over the solution polymerized and merely cross-linked (and not subjected to any chain transfer agent) latices of the prior art. Through the utilization of an emulsion polymerization process in conjunction with the presence of a chain transfer agent, it has been determined that the inventive polymer latex offers several novel and highly unexpected benefits. First, and most notably, the inventive latex effectively absorbs UV radiation and provides optimal colorlightfastness with a low amount of UV-absorbing monomer and a high amount of acrylate comonomer, particularly when applied to fabrics. Generally, one of ordinary skill in this art would expect that increased weight ratios of UV-absorbing monomer would improve UV-absorption and colorlightfastness. Surprisingly, such a belief is not the case here.
Second, the production of the best performing UV-absorbing polymer latex has been found to involve a manufacturing process utilizing a chain transfer agent (and not a cross-linking agent) during emulsion polymerization. Again, such a combination with the particular monomers and comonomers of the inventive latex produces the unexpected improvements in UV-absorbing properties and colorlightfastness, especially for fabric substrates. Chain transfer agents are unlikely candidates as additives within the particular polymerization processes simply because one of ordinary skill in the art would expect longer chain polymers to provide better lightfastness and UV-absorbing capabilities and chain transfer agents actually lower the degree of polymerization. Furthermore, such chain transfer agents are not normally used in emulsion polymerizations.
Third, it has been found that colorlightfastness characteristics can be optimized after polymerization upon producing the optimal surface tension of the latex. The ability of a latex to coat a substrate is highly dependent on the relative surface tension values of both the latex and the subject substrate. In order to thoroughly coat the desired substrate, then, it is necessary to modify the surface tension of the polymer latex to closely equal the surface tension of the subject substrate. The inventive polymer latex will generally obtain a much higher surface tension than that of the fabric or polymer film, as merely examples, on which the latex to be coated. The surface tension of the inventive polymer latex may thus be modified through the addition of wetting agents/surfactants in order to facilitate the desired coating process. Water-soluble wetting agents are one class of surfactants which may be employed in this respect, including lower aliphatic alcohols nonionic surfactants. The most preferred surfactants are those which are fluorinated, such as Zonyl(trademark), available from DuPont, and Fluorad(copyright), available from 3M. Some more examples of suitable surfactants as well as a further discussion of this aspect of the inventive method are presented in U.S. Pat. No. 5,629,365, to Razavi, as entirely incorporated herein by reference above.
Fourth, it has been found that the UV-absorbing monomer, when polymerized with the acrylate comonomer, and subsequently coated onto a substrate surface, provides improved UV-absorption and colorlightfastness when compared to coatings of conventional UV monomeric absorbers utilized alone and at equivalent UV-absorbing group concentration (i.e., the same amount of UV-absorber of prior art coatings as the amount of UV-absorbing monomer present in the inventive polymer latex). Such a result is very surprising and unexpected, considering the relative inexpensive cost of acrylate comonomers as compared to the cost for UV-absorbing monomers and/or compounds.
Last, it has been found that supersonic air post-treatment special processing of pile fabrics coated with the inventive UV-absorbing polymer latex improves colorlightfastness performance. Such supersonic air treatments include those disclosed in U.S. Pat. No. 4,837,902, U.S. patent application Ser. Nos. 08/593,670 and 08/999,638, and PCT Application PCT/US97/16,415, all to Dischler. Such treatments allow the individual pile fibers to xe2x80x9cstand upxe2x80x9d and/or open up improving colorlightfastness. The prior art teachings do not permit such extensive and beneficial characteristics.
The resultant inventive polymer latex comprises at least one UV-absorbing monomer and at least one vinyl functional comonomer. Generally, any proportions of these two constituents will provide effective ultra violet protection and colorlightfastness. However, it has been found that, surprisingly, ratios of amounts of the two monomers from about 10% UV absorbing monomer/90% vinyl functional comonomer to about 85%/15%, provide optimum results with regard to the aforementioned desired properties. From a colorlightfastness/cost effectiveness perspective, a more preferred range of ratios encompasses from about 50%/50% to about 25%/75%. Most preferred, again from a position that the UV monomers are very expensive and colorlightfastness of the latex on the fabric substrate is highly desired, is a range from about 40%/60% to about 35%/65% UV monomer to vinyl-functional monomer. Improved ultra violet protection seemingly would be afforded with a greater amount of such a UV-absorbing monomer within the final polymer latex product; however, upon control of the final molecular weight distribution through the utilization of a chain transfer agent in a batch emulsion polymerization process allows for lower amounts of expensive UV-absorbing monomer to be used in order to obtain the most beneficial characteristics. As discussed further below, a batch procedure (which entails the complete addition of all components simultaneously) provides unexpectedly favorable UV absorbing and lightfast characteristics as compared to a polymer latex formed through a semi-batch process (periodic additions of components over time). Even so, the inventive latex may be formed in any manner, with a batch procedure the preferred specific method.
The inventive polymer latex also required a certain solids content to permit optimum results. In particular, a rather low level of solids is desired. For instance, the entire polymer latex should have an average solids content (per individual polymer) of from about 15 to about 55%, preferably from about 20 to about 45%, and most preferably from about 25 to about 33%. These levels provide beneficial convenience in handling and resultant properties as well as better overall stability within the final polymer product.
Any benzotriazole- or benzophenone-containing vinyl-functional monomer may be utilized within the inventive polymer latex and method of making such a latex as what is described as the UV-absorbing comonomer. Such monomers are well known as providing effective UV-absorption for myriad surfaces, particularly on fabric substrates. The preferred UV-absorbing monomer is 2-Hydroxy-5-acrylyloxyphenyl-2H-benzotriazole (sold by Janssen Pharmaceuticals under the tradename Norbloc(trademark) 7966). Other particularly useful specific UV-absorbing monomers within this invention include 1-(3-benzotriazol-2-yl-4-hydroxyphenyl)-ethyl ester acrylic acid (manufactured by Hoechst Celanese), 2-(2-methacryloxy-5xe2x80x2-methylphenyl) benzotriazole (manufactured by Polysciences, Inc.), and 2-hydroxy-4-acrylyloxyethoxy benzophenone (sold under the tradename Cyasorb(trademark) UV-416 by Cytec, Inc.).
The vinyl-functional comonomer encompasses any non-UV functional group (benzotriazole, benzophenone, and the like) containing monomer having a vinyl functionality. Any such monomer can be employed within this inventive polymer latex; however, particularly preferred are those based on acrylic acids, including acrylates and methacrylates, and most particularly the following: butyl methacrylate, butyl acrylate, methyl methacrylate, butyl methacrylate, ethyl-hexyl methacrylate, lauryl methacrylate, isodecyl methacrylate, methacrylic acid, and n-hexyl methacrylate. Most preferred is butyl methacrylate, particularly in combination with 2-Hydroxy-5-acrylyloxyphenyl-2H-benzotriazole. The use of any of these or mixtures of these comonomers is dependent upon the desired stability and/or handling properties for the entire inventive polymer latex.
The term chain transfer agent means a compound which functions both as a polymerization inhibitor and initiator. Basically, after polymerization begins, such a process continues either until the supply of available monomer is depleted or until some type of inhibitor acts upon the polymerized compound. A chain transfer agent reacts with a polymerized compound by capping the reactive portion of the reactant (i.e., comprises a free radical or an ionically charged moiety) thereby effectively inhibiting polymerization of that one particular compound. However, such a chain transfer agent also has the ability to start polymerization in a previously unreacted monomer by displacing an electron or a leaving group. Such an agent is unique to addition polymerization processes, especially within such processes utilizing batch techniques. Any chain transfer agent may be utilized within the inventive method; however, preferred is 1-dodecanethiol. Other chain transfer agents workable within this invention include, without limitation, thiophenol, hydrophobic polymercaptans, and hydrophobic halogenated compounds. The chain transfer agent (CTA) should be present in amounts of grams CTA per grams aggregate UV monomer and vinyl-functional monomer of from about 0.002 to about 0.050%; preferably from about 0.008 to about 0.018%; and most preferably at about 0.0175%.
The polymerization initiator may be any of the well known compounds which perform such a function. For instance, peroxide, persulfate, and ultra violet light are only a few of the many potential compounds and manners of catalyzing the polymerization process within the inventive method. Preferred are coupled persulfate/bisulfites and azo(peroxides).
The textile fabric utilized within the inventive process may comprise any synthetic or natural fiber or blend of such fibers. As merely examples, and not intended as limitations, the textile fabric may be constructed from fibers of polyester, nylon (-6 or -6,6), cotton, polyester/cotton blends, wool, ramie, lycra, and the like. The preferred substrate is made of polyester, and most preferably polyethylene terephthalate yarns. Also, the textile fabric may be of woven, non-woven, or knit construction. Knit is the most preferred.
The application of the latex to a substrate may be accomplished through in situ formation of the inventive latex on the substrate surface or through any well known coating or impregnation procedure. Included, without any limitation intended, within this step are exhausting from a liquid formulation onto a fabric substrate, dipping/padding, knife coating, spraying, roll coating, foam coating, and the like. Particularly preferred is an exhaustion procedure from a liquid to a fabric substrate within a dye jet process.
To the subject substrate, any number of additives may be added either pre- or post-application of the inventive latex. For instance, when a fabric substrate is utilized, any standard textile additives, such as dyes, colorants, pigments, softening agents, antioxidants, flame retardants, rheology agents, soil redeposition agents, and the like, may be applied to the fabric surface. Cross-linking agents are not desired within the inventive latex. Cross-linking, which is a totally different process from chain transfer, would result in an uneven molecular weight distribution within the inventive latex such that the desired colorlightfastness characteristics, as well as ease of handling and application, would not be attainable. Particularly desired as optional finishes to the inventive fabrics are soil release agents which improve the wettability and washability of the fabric. Preferred soil release agents include those which provide hydrophilicity to the surface of polyester. With such a modified surface, again, the fabric imparts improved comfort to a wearer by wicking moisture. The preferred soil release agents contemplated within this invention may be found in U.S. Pat. Nos. 3,377,249; 3,540,835; 3,563,795; 3,574,620; 3,598,641; 3,620,826; 3,632,420; 3,649,165; 3,650,801; 3,652,212; 3,660,010; 3,676,052; 3,690,942; 3,897,206; 3,981,807; 3,625,754; 4,014,857; 4,073,993; 4,090,844; 4,131,550; 4,164,392; 4,168,954; 4,207,071; 4,290,765; 4,068,035; 4,427,557; and 4,937,277. These patents are accordingly incorporated herein by reference.
The inventive composite may be utilized for any substrate which functions as a covering, particularly from sunlight, or which has a surface which is colored and subject to ultra violet radiation degradation. As merely examples, then, the inventive latex may be applied to apparel, automotive upholstery, furniture upholstery, drapery, napery, tents, awnings, plastic bottles and/or containers made from polypropylene, polyethylene, polyurethane, polyethylene terephthalate, and mixtures thereof, and the like.
The preferred embodiment of the inventive method and composition is set forth in the following EXAMPLES.
Production of the Inventive Latex